The term high resolution is used to mean the ability to correctly portray or reproduce wide ranging dynamic signals, both in relationship to their correct peak values and, very importantly, in the ability to clearly separate extremely low level details of sounds from each other and from system background noise. As will be shown, these qualities are only realizable in systems characterized by minimal signal energy storage and consequently by minimal time-displacement distortions.
These distortion effects may often only be discerned by a discriminating listener using very high quality equipment. Average audio equipment either generates on its own or is prone to so much external distortion that the effects to which the invention is directed are often completely masked. Thus, it should be borne in mind throughout that these distortion effects are in the area below 0.01%, whereas the art is accustomed to dealing with distortions of above 0.1%. However, it is believed that these time-displacement distortions despite their low numerical percentage, give rise to very noticeable listening deficiencies.
If one considers the nature of sound as being "periods of energy" and "periods of silence", the importance of the "periods of silence" immediately becomes apparent. Anything that "fills in" a silent period is, therefore, a form of distortion. It has been discovered that this type of distortion, which is termed time-displacement distortion, is extremely noticeable, even in very small amounts. It has either not been recognized by the prior art or it has been ignored.
There are two types of time-displacement distortion in speaker systems that contribute to the system "noise floor": that due to stored energy in components, i.e., speakers, resistors, capacitors and inductors; and that due to the infusion of radio frequency (RF) energy into the system from the environment. For example, a capacitor that cannot release its stored charge rapidly introduces time-displacement distortion. Similarly, the resonance reaction of an unrestrained speaker is added to that of the sound being produced and generates time-displacement distortion. Further, a speaker is a generator in its own right and ambient sound or sounds from adjacent speakers produce back electromotive force (EMF) signals which are time-displacement distortions. In the area of RF, it is believed that the infusion of this energy interacting in a sub-audible way, is a major contributor to the noise floor of the system and results in an inability to reproduce very low level sounds, either in the presence of, or immediately following, high level sounds. The practical minimum noise floor is that set by the intrinsic electronic noise due to the components in the system. Anything that adds to the system noise floor degrades the system resolution and is a time-displacement distortion. As will also be seen, both types of time-displacement distortions are generally present.
All high fidelity stereo systems use at least two loudspeakers that are connected by a pair of wires to a power amplifier. The lengths of the connecting wires may range from three feet to thirty feet, depending upon the installation, and as such these wires may act as antennas for radiated electromagnetic waves. In urban environments especially, speaker connecting wires may pick up AM, FM, TV and CB signals. Some of the RF signals may be demodulated by nonlinear elements, such as very poor mechanical connections, and result in a clearly-audible gross form of interference. Obviously, RF interference of such magnitude demands corrective action. However, not all RF signals are demodulated to such a degree, or demodulated at all for that matter. While they may not therefore result in audible interference, they represent signal energy that is added to the noise floor and which adversely affects the amplifier and other circuitry by seriously restricting the dynamic range of reproducible signals. In speaker systems of high resolution and accuracy, such effects are quite noticeable.
One prior art solution to reducing gross RF interference has been to enclose the speaker leads with a shield connected to the amplifier "ground" and thereby prevent the infusion of the energy. In an ideal environment, this may be adequate. However, as a practical matter, it is difficult to obtain consistently good RF ground connections and, as will be seen, any failure to do so will increase time-displacement distortion even though ameliorating the gross interference.
In instances of such gross RF interference, various types of filters have been used, such as line filters for removing extraneous RF and other noise from the power lines supplying the system. Some filters have been sold for insertion between the amplifier and speakers for removing gross RF interference. As will be seen, these types of filters are totally unsuitable for use in high resolution speaker systems since their inherent characteristics actually contribute to the generation of the time-displacement distortions which this invention is intended to eliminate.
What the prior art has not realized is that even when no interference is audible, the infusion of RF energy into the system and on the connecting wires between the speaker and the amplifier degrades the clarity and dynamic range of the audio system. These time-displacement distortion induced reductions in fidelity are often quite substantial and give rise to very noticeable, albeit subjective, feelings of "mushiness" and "compression" in the reproduced audio information.
Most speaker systems divide the audio spectrum into two or more frequency bands by means of so called crossover networks. Signals in the different frequency bands are applied to individual speaker drivers that are optimized for those particular frequencies. It is well known that the size of a speaker required to move a given amount of air is in proportion to the wavelength of sound. Since the wavelength of sound increases with lower frequencies and decreases with higher frequencies, the size of a low frequency speaker driver is much greater than the size of a speaker driver designed to reproduce signals in the middle or higher registers. Similarly, it is well known that the peak-to-peak motion of a speaker driver required to produce a given sound pressure is inversely proportional to frequency, for any given size speaker driver. Consequently, as is well known, the distortion produced by speaker mechanical and magnetic nonlinearities also increases with lower frequencies. While many techniques have been used to improve the linearity of low frequency speaker drivers, distortion below 100 Hz is still very high--as much as five to twenty percent in most instances.
A major, generally unrecognized, distortion factor is that due to back EMF interactions between the low frequency speaker drivers, which are especially prone to high distortion, and the midrange and upper frequency drivers. This results in time-displacement distortion in that some of the higher order distortion products, generated by the low frequency speaker driver, are added to the drive signals supplied to the higher frequency speaker drivers. In the same way, audio signals that impinge on the speaker cones, cause the speakers to act as microphones and in turn to produce back EMF electrical signals which, when added to the electrical drive signals, result in time-displacement distortion.
The back EMF's of the speakers should ideally be suppressed to preclude interactions with other speakers and components. In accordance with an aspect of the invention, this is accomplished with frequency-independent energy dissipation means, generally in the form of resistors, coupled in the electrical circuit of the speaker. As will be seen, these back EMF current shunts have values ranging from 1.5 to 5 times the impedance of the speaker drivers, depending upon the characteristics of the speakers and the environment. Since the resistors are frequency-independent, out-of-band back EMF energy dissipation is obtained, which is believed to be the major factor in the improvement observed over prior art systems with crossovers. The back EMF shunts will, of course, generate heat since they dissipate energy. Conventional speaker "loading" devices, i.e., inductors in crossover networks, are frequency related and therefore ineffective against out-of-band back EMF energy and, of course, can not dissipate such energy. Crossover networks, therefore, change the "loading" on their separate speaker drivers because these loading effects are frequency related. Use of the back EMF shunts taught by this invention, in conjunction with the crossover networks, substantially eliminates such changes in loading effect by dissipating the time-displaced energy.
The so-called "Bi-Amp" (also "Tri-Amp") configuration was an attempt to overcome many of the speaker loading problems associated with crossover networks. In these multiple amplifier approaches, separate amplifiers were used for different ranges of frequencies and in turn drove their associated speakers. Such systems were capable of much better control of speaker loading and were also free from the phase problems associated with passive crossover networks. Such an arrangement minimized the back EMF interactions of the speaker, although it was apparently not generally recognized. Their use is obviated by the system of the invention.
The art has also not apparently appreciated the additive nature of many small distortion producing elements on high resolution speaker systems. Mechanical junctions that are clearly rectifying in nature are, of course, obviously bad. But, as this invention shows, all mechanical connections are suspect and should be appropriately treated. Further, when the dynamic signal handling capability is increased by application of the principles of the invention to reduce time-displacement distortions, the effects of the previously hidden, i.e., masked, minor distortion producing elements become all too clear.
Capacitors are prime examples of elements that can be major sources of time-displacement distortion, especially due to RF energy infusion. Thus, a capacitor that is specified herein as an RF capacitor, and indicated in the drawing with curved lines rather than straight lines, needs to be "linear", that is, exhibit a linear voltage-charge relationship, at least up to 20 MHz and must have a low dielectric energy absorption. It will also be clear from this discussion that audio capacitors should also be linear and exhibit low energy absorption. This latter characteristic is directly related to the ability of the capacitor to give up its charge quickly. Capacitors that do not exhibit this characteristic introduce energy storage which gives rise to time-displacement distortion of the signal and compression of the dynamic range of the system. The RF capacitors illustrated may be 0.001 to 0.02 microfarad mica, glass or high quality film types.
Prior art filtering attempts using capacitors to eliminate gross RF signal interference were counterproductive with regard to time-displacement distortions. Indeed the use of ceramic-type disc capacitors connected to "ground" would very seriously degrade a high resolution audio system by introducing time-displacement distortion due to their extreme non-linearity.
Every mechanical connection should be individually determined to be good or bypassed by an RF capacitor. Resistors should, of course, be wire wound or of equal quality. Carbon resistors are totally unacceptable because of their notorious susceptibility to changes in pressure, whether electrical or mechanical. Such changes increase the energy storage of the system and give rise to time-displacement distortions. The internal terminations of wire wound resistors are very important and should be bypassed, if there is any doubt. Once the concept of time-displacement distortion, either by electrical or mechanical energy storage of components or by RF energy raising the system noise floor is grasped, the need for careful attention to each potential distortion source is apparent.
It has also been discovered that even very small amounts of time-displacement distortions become much more noticeable in unsymmetrical networks, that is, in networks that do not electrically "look the same" to both polarities of audio signals. This phenomenon is believed due to unsymmetrical audio signals impacting less-than-ideal components and thereby emphasizing, in a differential way, the non-linearity. These effects are also seen in connection with the effects of infusion of RF signal energy. It has been determined, for example, that drawn wire has a preferential "direction" for minimization of distortion with audio signals, probably due to the molecular grain orientation determined in the drawing process and the inherently unsymmetrical nature of audio signals. While this phenomenon is not fully understood, the effect of reversing improperly oriented wire is clearly perceptible to discerning listeners.
The asymetrical nature of audio signals has a significant impact on crossover networks. A large reduction in time-displacement distortion can be achieved in crossover networks that are "split and balanced" to appear electrically identical to either polarity of signal.
It is thus apparent that the prior art leaves much to be desired with respect to high resolution loudspeaker systems. The invention, in its various aspects, recognizes and provides solutions for the major deficiencies of the prior art.